Highwall mining is a procedure for extracting coal or other mineral from a thin seam with remote control machinery which extends into the seam from a bench at the surface of a highwall. A highwall is a vertical or steeply sloped face of an excavation which has been cut on a mountain side or has been exposed by the digging of a pit in the course of a surface mining operation. The portion of the seam which has been removed by the mining excavation will leave a relatively flat and generally horizontal surface at the base of the highwall referred to as the bench. As a result, at the base of the highwall will be found an artificial outcrop of coal or other mineral underlying an overburden of soil and rock. From the outcrop a continuation of the mined seam will extend into the earth or under the mountain.
The creation of highwalls is in part due to the economic factors involved in strip mining. The cost of exposing and removing coal from a seam is related to the amount of overburden which must be removed to expose the seam. As the surface mining operation progresses, if the overburden progressively thickens, it eventually ceases to be economical to continue surface mining. This is because the amount of overburden material which must be removed per given amount of coal recovered reaches a ratio at which it is no longer cost effective to proceed. As a result, the seams of the coal remaining beyond the face of the highwall must be mined, if at all, by other techniques.
With seams of this type, the thickness is frequently too small to allow the mining to be economically carried out by miners working within the seam. Often, the seam may be no more than two to three feet in thickness. For this reason, the art of highwall mining is undertaken by the use of remote control machinery.
Early efforts to mine a coal seam from a highwall included the use of coal augers to bore circular holes in the coal seam. Auger mining was employed to mine coal seams from benches cut in mountainsides or left from surface mining operations. These auger devices, however, made poor utilization of the natural coal. Often the coal removed was contaminated with material from outside the seam since there was no effective method for guiding the advancing auger.
Other developments in highwall mining involved the use of continuous mining machines which were remotely controlled to proceed into a coal seam. These machines were followed into the mined hole by open conveyor vehicles which removed the coal to the surface. Such devices, as for example one referred to as the Push Button Miner developed by Joy Manufacturing Co., were only marginally effective and have been practically abandoned.
More recent developments in highwall mining machines have been more successful. One such machine is disclosed in U.S. Pat. No. Re 31,622 issued to Robert E. Todd. Machines such as those shown in Todd Reissue Patent No. 31,622 represent a substantial improvement over prior equipment and methods for mining thin seams from a highwall.
The Todd miner includes a cutter head which is driven into the coal seam by a rectangular compression beam which is assembled in sections behind the mining head and is thrust from a baseframe at a bench outside of the mined hole at the surface of the highwall. The mining head is similar to that of a continuous mining machine. Such machines permit deep entry into the coal seam, sufficient recovery of resources and economical production of coal. Guidance of the cutting heads of such machines has heretofore been a work of art, depending highly upon the skill and experience of the human operator. The efficiency and effectiveness of operator directed guidance has been limited by the difficulty in locating the boundaries of coal seams which may be hundreds of feet removed from the operator and in positioning the cutter heads within inches of such boundaries to mine the coal.
With the highwall mining of coal, as with any mining operation, it has been a general objective to extract from the earth the maximum amount of coal at the minimum cost. Costs in the mining of coal are usually directly related to the time during which equipment and labor is in use. Labor costs are commonly related to man hours of work, while equipment costs too can be related to the amortization of capital and maintenance over equipment life. Thus, a basic ingredient in the cost of produced coal is expressed in a certain unit of cost per unit time of operation. The coal produced during that unit time is thus produced at the cost of operation for that unit time. Thus, the general objectives in coal mining have been to increase the coal production per unit hour and to decrease the cost per unit hour to increase the amount of coal produced per dollar expended. This has been the ultimate objective of coal mining operators.
An additional objective in the coal mining industry has been the economic use of the mineral resources of the land. It is important to the owners of the land containing coal, to the owners of the mineral rights to the land, and to the nations concerned with conserving energy resources that the percentage of the coal recovered from the land by the mining operation be maximized. The technique of highwall mining has increased recovery of coal by providing, in many cases, the ability to mine coal which cannot be economically mined by any other process. This is due in part to its ability to recover coal from the seams which are too thin to be mined by any other process. Nonetheless, it is still important that the highwall mining process recover the maximum amount of coal which can be recovered by that process.
One occurrence in highwall mining which results in a reduced recovery of coal from a coal seam is a failure to mine close to the boundary of the coal seam. Accordingly, as a highwall mining machine mines a hole into a coal seam, a layer of coal will remain near the top and bottom of the mined hole leaving a greater thickness of coal than necessary adjacent these mined holes. This necessarily reduces the recovery percentage of the coal. Moving the cutters close to the coal seam boundary results in removal of a greater amount of coal, but also runs the risk of cutting beyond the boundary of the coal seam which will remove out of seam material along with the coal. When this occurs, the rock mixed with the coal, referred to as the ash content of the coal, can greatly reduce the value of the coal on the market. The value reduction often will more than offset the value of the coal which would be lost by leaving a greater thickness of coal in the seam. By taking greater care to mine close to the boundary without crossing the boundary, a higher recovery of cleaner more valuable coal results, but the mining process is slowed and accordingly the cost of the coal per unit ton of coal mined is increased due to the overall slowing of the mining operation.
A problem which has been encountered in high wall mining has been the lack of knowledge of the boundaries of the coal seam adjacent to and in advance of the cutter head. The employment of coal thickness sensors of various types has been common with highwall mining machines developed in the last ten years. One type of sensor used has been the sensitized pick which is capable of detecting differences in physical properties of the material at the mined surface to distinguish between coal and the material outside of the seam. Other such sensors have, for example, included natural radiation sensors, which measure the strength of the natural gamma radiation originating from the surrounding clay based rock. This radiation is attenuated by the thickness of the coal. Such sensors, when positioned on the highwall mining machine cutter head, are capable of producing a measurement signal representative of the thickness of the coal between the boundary of the mined hole and the boundary of the coal seam.
The positions of such sensors, however, do not lie ahead of the advancing cutter but must necessarily be within the hole already mined. At best, the positioning of such sensors is at the leading edge of the cutter within the mined hole. However, the cutting site is not the most ideal environment for the proper operation of such scientific instruments even those adapted for heavy industrial uses. The risk of mechanical damage to the sensors in this region, as well as the interference with their accurate measurements of coal, dust, and debris, usually requires that the sensors be positioned somewhat behind the leading cutting edge of the cutter head. Accordingly, the fact that the sensors would thus be located several feet behind the cutting interface diminishes the value of the measurements in guiding the actual positioning of the cutters, since prediction rather than accurate measurement of the coal seam interface must be relied upon.
The ability of prior art techniques to provide an accurate prediction of the coal seam boundary so as to provide a basis for guidance of the machine cutter head in the mining of the seam ahead of the cutter head has been inadequate and unsatisfactory. Accordingly, there has been a need for an improved manner of predicting for purposes of effectively controlling the movement of the cutters and cutter head of a highwall mining machine within the coal seam.
Certain prior art techniques have been developed for guiding mining machines to cut near the boundaries of coal seams. In the art of longwall mining for example where the cutter heads of mining machines are precisely positioned on firmly anchored rails and thrust against the coal from firmly anchored and precisely positioned beams and columns, some semblance of program control has been attempted. With such programs, information from mining pass measurements has been used to determine the extensions of the coal seam boundaries and thereby guide the machine. Such techniques have not been suggested nor been regarded as at all practical for highwall mining machines where the cutter heads may be several hundred feet from the bench on which the power head driving the cutter head is anchored and where cutter heads are not in physical contact with a previous cut. With the cutter head linked through a multisection articulating compression beam to the power head, and where the mined hole is not contiguous with the previous mining pass but is rather through a totally separate mining hole, predicting the seam extensions and locating the cutting equipment has lacked enough accuracy to avoid errors in the guiding and controlling of the mining machine. Accordingly, there is a need in the prior art for providing a method for not only mapping the boundaries of a coal seam in such a way that the information would be useful for guiding highwall mining machines, but also to provide a manner to practically use the information in a way to overcome the uncertainties involved in extrapolating and predicting the course of the coal seam ahead of the cutter head in a separately mined hole.